1. Field of the Invention
This invention relates generally to floating hydrocarbon storage vessels connected to subsea wells and particularly to such storage vessels having a turret anchored to the sea floor with the storage vessel weathervaning about the turret.
2. Description of the Prior Art
In mooring systems for floating vessels used in the development of offshore oil resources, a turret anchored to the sea floor and mounted within an opening in the hull of the floating vessel is often used where the vessel weathervanes about the turret. Product risers extend from the subsea wells to the turret and are connected to pipelines in a lower portion or shaft of the turret for transfer of hydrocarbon product to storage areas of the vessel. The product risers which extend to subsea wells, or manifolds for such wells are often supported by a spider buoy which is releasably connected to the turret; pipeline connections are made between the spider buoy and turret for the transfer of product. Hydrocarbon-based gases which can be released by these pipeline connections are highly explosive if a certain gas/air mixture is present. This gas/air ratio is between 1% and 17% hydrocarbon gas to air. The lower turret shaft generally has an open volume within the ship that is largely confined on virtually all sides. This large volume combined with the tight confinement (lack of ventilation) has the potential of generating very high blast over pressures were an explosion to occur.